My kitchen remodel is in progress. After ripping out existing cabinets we found that the plumbing for the sink drain cuts through a total of 7 studs, more than 50% through each (see photos). Contractor and I were originally worried that these were load supporting -- why else use 5 together? But contractor went into the attic, and said based on rafters this is not load bearing. Consequently they are going to leave the existing situation. Next room has vaulted ceilings, and there is quite a bit of structure in the attic to support/implement that.

Should I replace the studs and somehow route the plumbing around, or is it ok as pictured?

The new sink will be in the same location ( a peninsula which comes out at 45 degress from the end of the wall there), and so the PVC that is there now, pictured, is still needed to reach the basement stack. I guess. This is outside my area of expertise (software eng) so I appreciate and ideas/opinions/thoughts. Thanks.

Your structure is compromised. That many adjoining studs is carrying weight to some point in the basement from someplace above. I would certainly restore the structure!

Is there no possible realignment of your project to replace the lost support? There does not look like a place to just sister some new 2x4s onto that mess. Can you access the same spot by bringing the drain up from the basement on the right side of the clustered 2x4s then re-route back into the basement stack? If you have to, build a plumbing soffit downstairs to hide the fix (Make it look like a column).

It's a safe bet to assume the contractor is wrong, repair that mess and bring the pipe out of the wall, it will be at the back of the cupboard below the shelf, no harm .
Any time you find more than one stud nailed together, it will be barring. You have a beam running to the right to support the ceiling in the other room, my guess.

The basement is not finished. The pipe going into the basement pictured above is the one closest to the camera in this picture below. The previous, and new, sink, will be to the left of that joist, you can see the water pipes. The five studs come down near the drain's entrance to the basement, so about 3.5 feet from the gray support beam you see.

It is hard to see in the original picture, but the drain also goes up, inside the wall, to the attic. I guess whatever new solution we come up with will have to maintain that. The sink, being in a peninsula not abutting the wall, some plumbing in the wall will be necessary. Maybe it can be in the new 12" extension to the wall there (original picture, with no drywall)-- added because the new cabinets will extend further along that wall.

Your pics make it clear you can re-route the drain to the right in the stud bay. Here is a pic of my concept. You can lose that destroyed stud issue and have an easy way to go. Just punch up fro the basement about 24" to the right and BINGO!